In recent years, the domestic and international researchers and producers in the field of refining catalysts has devoted themselves to improve the performance of FCC catalyst by means of molecular sieves with various types of structure in order to increase the yields of light olefins in FCC plants. This is because the primary process for traditional production of ethylene and propylene, i.e. steam pyrolysis, is performed by the free radical reactions, wherein the temperature is high and the requirement to feedstock is rigorous. In contrast, catalytic pyrolysis for producing light olefins has relatively lower cost and is the hot spot in the domestic and international research for increasing light olefin capacity. Functionalized shape-selective molecular sieves deserve wide attention as the major active constituent in such kind of catalytic converting catalyst.
Since USA Mobil Company has developed the ZSM-5 zeolite molecular sieve (U.S. Pat. No. 3,702,886) in 1972, it has been widely applied in petrochemical processes, such as shape-selective cracking (CN 1872415A), alkylation, isomerisation, disproportionation, catalytic dewaxing, etherification and etc. of hydrocarbons, due to its properties of relatively high silica to alumina ratio, unique pore structure, and excellent thermal and hydrothermal stabilities. In particular, adding ZSM-5 zeolite into normal catalysts or aids for catalytic cracking enables to significantly increase the yields of light olefins and the octane number (U.S. Pat. No. 5,997,728).
However, the ZSM-5 zeolite molecular sieve is readily to be deactivated under the hydrothermal condition of the catalytic cracking, and thus the stability and selectivity are both affected. Therefore, a number of researches have been carried out about the modification to the ZSM-5 zeolite molecular sieve.
In U.S. Pat. No. 4,399,059, diammonium hydrogen phosphate or ammonium dihydrogen phosphate is mixed with NH4-ZSM-5 and dried, and the mixture is calcined at 500° C. so as to produce a phosphorus-modified ZSM-5 zeolite, which makes it possible to significantly improve the selectivity of para isomer product when used in the isomerisation reaction of xylene.
U.S. Pat. No. 5,171,921 discloses a ZSM-5 molecular sieve modified by impregnating with phosphorus compounds. Such modified molecular sieve may be used as the catalytically active constituent that converts olefins or aliphatic hydrocarbons into C2-C5 olefins.
U.S. Pat. No. 3,972,382 and U.S. Pat. No. 3,965,208 disclose that the reaction selectivity of the HZSM-5 is improved after being modified with trimethyl phosphite.
CN 85102828 reports modifying the ZSM-5 molecular sieve using a method by impregnating and evaporating, and the phosphorus-modified molecular sieve has a substantially improved activity in the shape-selective catalysis for preparing para-ethyltoluene by alkylation of toluene with ethylene.
CN 97120271 reports a phosphorus-containing faujasite as hydrocarbon cracking catalyst, said phosphorus-containing faujasite is prepared by uniformly mixing faujasite with an aqueous solution of a phosphorus-containing compound followed by drying and calcining at 450-600° C. for more than 0.5 h, and has a relatively good catalytic activity.
CN 98117286 reports a phosphorous zeolite comprising 90-99.9 wt % of aluminosilicate zeolite and 0.1-10 wt % of phosphorus based on P2O5, which has higher hydrocarbon converting activity, higher diesel selectivity, and better resistance to vanadium, nickel and other heavy metal.
CN 1211469A and CN 1211470A report a molecular sieve composition with increased yields of propylene and ethylene, characterized in that, it is obtained by adding a five-membered ring molecular sieve into an aqueous solution of a compound comprising phosphorus and alkaline earth metal ions and/or transition metal ions, mixing until homogeneous, and allowing impregnation reaction, said composition is composed by 85-95 wt % of five-membered ring molecular sieve, 2-10 wt % of phosphorus based on oxide, 0.3-5 wt % of alkaline earth metal based on oxide, 0.3-5 wt % of transition metal element based on oxide.
CN 1072031C and CN 1072032C also report a five-membered ring molecular sieve composition with increased yields of propylene and ethylene(especially ethylene), which is composed by 88-95 wt % of five-membered ring molecular sieve, 2-8 wt % of phosphorus based on oxide, 0.3-3 wt % of alkaline earth metal based on oxide, 0-3 wt % of transition metal element based on oxide. The preparation method of said composition is one step impregnation method, wherein the procedure is not only relatively simple and well reliable, but also has a substantially increased ethylene yield.
The yields of light olefins, in particular propylene, of the catalytic cracking catalysts used in the industry in prior art are not sufficient to satisfy the practical requirement of the manufactures of refining and chemical engineering. Therefore, it is quite meaningful to substantially increase propylene yield. So far it seems that modification to the functionalized shape-selective molecular sieve would be the most efficient route to increase the yields of light olefins in catalytic cracking.
However, when a second modifying constituent is introduced into the traditional phosphorus-modified molecular sieve via ion exchange, the phosphorus element in the molecular sieve would be largely washed away, and thus the purpose of modifying the molecular sieve with two components can not be achieved. As a result, the stability and catalytic activity of the molecular sieve are affected.